1. Field of the Invention
The present invention relates to a developing device which is used in a copying machine, a printer or the like, and more particularly to a developing device which enables an acquisition of an image of high image quality by adjusting a developing bias at the time of developing and improvement of an image forming apparatus which uses the developing device.
2. Description of the Related Art
Conventionally, in an image forming apparatus such as a copying machine, a printer or the like which adopts electrophotography, there has been known a developing method which uses a two-component developer containing toner and carrier and a photoreceptor is directly developed with a magnetic brush of a developer. In such a developing method, when the developing is continuously performed, the density of toner in the developer or the like is changed and hence, an image quality of an output image is changed.
To the contrary, even when the change of toner density is simply detected and the toner is replenished in response to the detection of the change, it is difficult to obtain the proper image quality due to a change with time in the fluidity and the charging property of the developer per se.
Further, in general, to obtain the high image quality, there has been known a technique which superposes an AC component on a DC component which constitutes a developing bias. By superposing the AC component to the developing bias, it is possible to obtain an advantage that the image density of a solid portion (a matted portion), fogging of a background (a non-image portion), fine-line reproducibility, graininess and the like are enhanced. Accordingly, there has been proposed a method which aims at the maintenance of image quality by changing amplitude and frequency of an AC component of a developing bias in conformity with the use history or the like of the developing device.
In the method described above, the frequency of an AC component of a developing bias is changed based on the input image information, the output image information, and the use environment information of a developing device. By preliminarily obtaining the relationship between the frequency and the gradation of an output image (density level of an output image) and by changing the frequency corresponding to the change of the charging property, it is possible to obtain the image which sufficiently ensures the gradation of the output image. Further, according to the method described above, the use environment information of the developing device includes the use history information such as the number of developing times, the change of the charging property of the toner and the like.
Further, according to another technique, the use history of a developer is estimated based on the number of times that the developing is performed, a developing time, a charging property change of a photoreceptor and the like, or the use history of a developer is detected based on the change of toner charging property or the resistance change of a carrier, and an amplitude of an AC component of a developing bias is changed in response to an estimated value or a detected value to perform the correction with respect to a change with time in the fluidity of the developer, whereby the transfer performance of the toner can be maintained.
Surely, according to the above-mentioned methods, compared to a method which holds the AC component of the developing bias at a fixed value, the gradation and the transfer performance may be maintained for a long period. However, in these methods, a developing amount when the developer per se is changed (for example, a developing amount being different substantially due to the change of easiness of mobility of toners in the developer) and the relationship between banding (stripe-like density irregularities which appear in a half tone portion of an image) or the like and the developing bias are not taken into consideration at all. Further, even when a current change of the developing bias is detected as described above, the method merely detects the influence which are relevant to both of the developer and the photoreceptor and does not directly detect the change of the developer thus giving rise to a drawback that the maintenance of proper image quality becomes insufficient.
Usually, along with the use of the developer, a rate of toner amount in the developer may be changed, the toner and the carrier may be changed, and the degree of concentration of the developer may be changed. Accordingly, it is extremely important to properly grasp changes of characteristics of the developer and to maintain the proper image quality.